> If you read my "Science and Society" article, you'll note SPS
> launched from Earth may compete economically with base power
plants
> (nuclear, coal - capital costs about $1-4/watt) with current
launch
> costs of $10,000/kg (i.e. we don't necessarily need lower launch
costs)
> on three conditions:

> * transmission efficiency is high enough (> 40%?)

That's easy, I think

> * solar panel costs come down to about the $1/peak watt level
> (terrestrial solar panels aren't far; solar panels designed for
space
> use are 100's of times higher right now)
> * (this is the key one) - the mass per kW in space can be kept
below 0.1
> kg/kW.

As for these two, I don't think that it is possible do both at the
same time. A major problem is not just launch costs but launch
_environment_, It has to be tough enough to survive ascent. Even if
it can, the acceptance tests alone will put you over budget even if
the arrays are free.

> But that third constraint is the toughest. Panels have been
deployed in
> space with about 20 kg/kW; commercial modules seem to be available
at
> about 5 kg/kW.

I had HS 601HP panels quoted at something like 200W/kg. In any
case, photovoltaics is not the way to go at the scale of an SPS; the
best way to go is thermodynamic generators. The heat efficiency of
a modern Brayton Cycle powerplant is on the order of 80%, and if
collection efficiency is high enough (95% should be attainable),
about quadruple the power can be collected from the same "wing
area". TDG collectors don't need to be round parabolas either, they
can be made square (although square ones would be tough to deploy.)

> It's not clear how light we can go. The constraints in
> space are very different than on the ground, where rigid strength
is
> essential in the presence of wind and rain and other assaults. You
write
> that an SPS has to "resist [...] maneuvering and solar pressure
induced
> torques" - a very light one can rather use such things to move
itself
> around.

That much is obvious; I think I mentioned that about the solar
pressure aided co-orbiting power stagion for a space colony.

> If we have to launch from earth, all we need do is get to low
> orbit, and then the light SPS components can move themselves under
their
> own electric power, using solar electric propulsion technology,
with
> very minor propellant requirements.

Not exactly; this involves deploying the collectors on LEO, and
there are a whole raft of problems, including, but not necessarily
limited to:

- oxidization from atomic oxygen in the thermosphere (LEO
environment, an aspect of which is hard to notice unless you're a
solar sail or SPS.)
- atmospheric drag
- eclipses reduce overall SEP efficiency and force maneuver
constraints
- gravity gradient torques make SPS hard to point properly at the sun
- long time to get into service

> Once in place, say at GEO, they can
> use solar sail techniques for station-keeping - there are plenty
of
> mathematical analyses around that'll help you do that very
precisely
> with whatever the thrust you happen to have - which is what you
seemed
> to suggest in your notes as well...

Not so much for station keeping; it would be useful for attitude
control, but station keeping would be harder because you'd need to
balance N-S translations with prograde/retrograte (E-W)
translations. Also, with TDG plants, the pointing has to be kept
within 3deg, which would require a secondary PVA or solar sail
structure to use solar pressure. A more flexible method would be to
use SEP Hall effect thrusters.

> For reference, the Planetary Society's solar sail uses thin mylar
of
> 0.02 kg/m^2, the equivalent of less than 0.1 kg/kW if they
included thin
> film solar cells of 20% efficiency. So that number may not be
> technically impossible - with nanotechnology, there's still a lot
of
> "room at the bottom".
>
> But if 0.1 kg/kW isn't possible, 1 kg/kW means we need launch
costs of
> $1000/kg, ten times below today's. If we can't actually do better
than 5
> kg/kW that seems to be commercially available now, then we'd need
launch
> costs of $200/kg.
>
> In other words, it's not a factor of 2400, but a factor of 50 that
may
> be necessary. At least by those numbers.

These are total costs, not launch costs alone. Assuming Bluestar's
LEO cost estimate of $200/kg, and current solar wing PVA at 200W/kg
(which does not includes booster adapters, propulsion, AACS, flight
control and "parasite power losses", which would be the spacecraft's
own operating needs), then yeah, $1/W might be possible. Oh, yeah,
I almost forgot, that doesn't include the GEO package, which costs
$2500/kg. I calculated that to include a space tug, the propellant
costs for it (which consist mostly of Bluestar's aforementioned
$200/kg, plus about $50/kg for the cost of the propellant and losses
from the tanks needed to carry it.) It included space station
rental, EVA and space crane assisted deployments of a GEO satellites
antennae and solar wings, and an expended upper stage to put it the
last 1500m/s to GEO from where the reusable space tug leaves off.
The the wheels of the Bluestar booster carrying the spacecraft
itself lifting off the runway to the time it was on station was
about four months (reality's is two months, but reality's vs.
Bluestar's puts many of the processing activities in an M6.5
cleanroom on the ground instead of at the space station. Total
campaign was about 12 months for a new type of spacecraft that
ascended inert on Bluestar and had all its deployments and hazardous
stuff installed in space.)

That is an optimistic scenario, by the way. We could and should be
doing this sort of thing right now. By itself, the fact that we
aren't makes it optimistic.

> It's not clear we have sufficient energy from other sources to
meet the
> needs of Earth-based industry through the end of this century.
With
> fusion we would, if a practical design for that were ever
economically
> competitive.

Actually, it's not so hard. JET was close to breakeven when it lost
it's funding. That's in Zubrin's book too. Also, an SPS becomes
increasingly useless the further away from the Sun you get;
ultimately we will go nowhere without nuclear power or an equivalent.

> David Criswell's lunar solar power proposal is another
> alternative

This doesn't stand a chance against earth based solar power. The
specific power of a solar collection system on the Moon wouldn't be
much higher than using the same technology on Earth. Plus, you'll
have to deal with the free space losses and tracking requirements
vs. a GEO SPS.

> Like your Mars colony, it would grow
> itself out on the Moon with little need for further input from
Earth.

There isn't much for hydrogen and carbon on the Moon; also, getting
oxygen involves baking rocks and then compressing the thin air from
them through several stages, a much more difficult process than the
Mars equivalent. It is a lot easier to build a colony on Mars than
the Moon. The Moon has a number of marketable resources. For Earth
it has He3, and for everyone else (including the hopefully beyond
nascent Mars colony), more titanium than you can shake a stick at.
Titanium is a staple of spacecraft structures; once refined and
basic annealed products (i.e. sheet, plate, billet, slab, bloom,
tube, etc.) are available from the Moon, Mars would rather get them
from there because ascent costs are much cheaper. Even if Mars has
enough of its own, it will be easier to get the lunar stuff from the
Moon to the asteroid belt than it will from Mars (i.e. by using mass
drivers and little tugs.)